Field-effect controlled power switching devices such as a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) or an Insulated Gate Bipolar Transistor (IGBT) have been used for various applications including but not limited to use as switches in power supplies and power converters. One example illustrating the use of MOSFETS in a dc to dc converter is given in FIG. 1.
The direction of current flow through the field-effect controlled devices operating as switches may be different in different operating cycles of power converters. In a “forward mode” of the field-effect controlled device, the pn-body diode at the body-drain junction of the field-effect controlled device is reversely biased and the resistance of the device can be controlled by the voltage applied to the gate electrode of the field-effect controlled device. In a “reversed mode” of the field-effect controlled device, the pn-body diode is forward biased. This results in a loss which is mainly determined by the product of current flow and voltage drop across the body diode. To minimize losses during reverse mode of the field-effect controlled device, i.e., maximize efficiency of the power supply or power converter, a shunting device, e.g., a diode, can be switched in parallel to the body diode of the field-effect-controlled switching device. Ideally, the shunting device should conduct no current when the body diode is reverse-biased and turn on at a lower voltage than the body diode when the body diode is forward-biased. To avoid unwanted inductivities and capacities associated with the required contacts and supply lines of additional devices, integrated power devices including e.g., a MOSFET and a diode have been proposed.
Commonly, mainly Schottky diodes have been used as integrated shunting devices. A Schottky diode is characterized by a low forward voltage drop of about 0.4 V at a given typical current, a low turn-on voltage of about 0.3 V, fast turn off, and nonconductance when the diode is reverse biased. For comparison, a silicon pn-diode has a forward voltage drop of about 0.9 V at given typical current and a turn-on voltage of about 0.6 V to 0.8 V. The losses during reverse biasing of a silicon MOSFET can, therefore, be reduced by connecting a Schottky-diode in parallel to the pn-body diode. However, to create a Schottky diode a metal-semiconductor barrier must be formed. In order to obtain proper electric characteristics for the Schottky diode, the metal used for the Schottky-contacts likely differs from the metal used for other structures such as Ohmic metal-semiconductor contacts. This can complicate the manufacture of the device. Further, the quality of a Schottky diode is usually affected by subsequent processes required for forming the MOSFET. In addition, Schottky diode rectifiers suffer from problems such as high leakage current and reverse power dissipation. Also, these problems usually increase with temperature and current thus causing reliability problems e.g., for power supply and power converter applications. Therefore, monolithically integrated power devices including Schottky barrier diodes can cause design problems.
For these and other reasons, there is a need for the present invention.